The present invention relates generally to pulse oximetry devices and methods. More particularly, the invention is concerned with a pulse oximetry system that provides enhanced signal quality information to the operator via signal sonification, in order to improve monitoring accuracy and availability.
Pulse oximeters are well known in the art. Typically, such devices comprise a sensor with light emitting device(s) and associated photodetector(s), attached to a monitoring device performing signal acquisition, analysis, and display/print functions. One particular example of a pulse oximeter is described in U.S. Pat. No. 5,842,981.
The signals acquired from a pulse oximeter are proportional to the tranmissivity of the biological tissues at the sensor. When visualized as photoplethysmographic waveforms, pulsations are visible occurring in synchrony with the heart rate. These pulsations result from the increased absorption of light occurring during passage of blood through the arterial system. Because the arterial pulsation is the result of systole in the heart, this rapid increase in absorption (decrease in detected light intensity) is referred to herein as the systolic phase of the signal. The intervening time between systolic phases, characterized by a relatively gradual decrease in absorption, is herein referred to as the diastolic phase. By choosing appropriate wavelengths of light, the plurality of oximetry signals can be interpreted to yield the percentage of saturation of the hemoglobin molecules with oxygen (SpO2). In the prior art, red and infrared pulsatile amplitudes, scaled by their respective mean light intensities, are combined in a ratiometric equation to yield a ratio related to SpO2.
The pulsatility of the photoplethysmographic waveform is very distinctive, resembling an inverted arterial blood pressure waveform even to the extent that a dichotic notch is often visible. (This similarity is commonly emphasized by inverting the photoplethysmographic waveform.) Like the arterial pressure waveform, the oximetry signals represent the hemodynamic activity of the cardiovascular system, rather than electrical activity of the heart like an ECG. Hence oximeters often provide a means of signal visualization to convey signal quality as well as physiological information (such as the pulse rate and rhythmicity).
Successful physiological monitoring, including pulse oximetry, depends upon acquisition of usable signals from the sensor(s). One problem associated with pulse oximetry is that signal quality can be highly dependent upon sensor placement and the condition of the underlying tissue. This problem is relatively worse in reflectance mode pulse oximetry, wherein signals are typically of lower intensity.
Reflectance pulse oximetry appears, however, to be the only viable method for in utero fetal pulse oximetry. The sensor is inserted into the uterus of a mother to noninvasively monitor the condition of a fetus, a mother, and a placenta. One particular example of a sensor designed for fetal pulse oximetry is described in U.S. Pat. No. 5,425,362. The sensor placement is made through the birth canal to reach a monitoring position on the fetus. This process and its outcome are difficult to satisfactorily visualize, even utilizing intrauterine imaging technologies such as ultrasound. Thus, fetal pulse oximetry represents a challenging scenario for signal acquisition in medical monitoring.
Clinical experience with fetal pulse oximetry bears this out. A recent study looking at 164 cases in which fetal oxygen saturation could be measured found that reliable signals were available only 64.7% of the time during the first stage of labor, and even less during the second stage of labor (Goffmet et al, 1997). Other studies have reported still lower availability. This percentage of monitoring availability is much less than experienced in clinical practice when using pulse oximetry in adults or even neonates, indicating the difficulty of fetal oxygen saturation monitoring and the need for further improvement.
Thus, it is important to provide the clinician assistance in assessing the efficacy of sensor placement by indirect means, but commercially available systems have failed to satisfy this requirement. The prior art suggests several possible solutions.
In one prior art fetal oximetry sensor, described in U.S. Pat. No. 5,247,932, electrical impedance is monitored near the sensor""s active components to detect whether the sensor is in contact with tissue. If both electrodes are bathed in amniotic fluid, the impedance is lower than when one electrode is in contact with wet tissue. A high impedance interface, however, does not guarantee that the tissue site is suitable for pulse oximetry, i.e., adequately perfused and free of interfering material. Nor does impedance measurement alone tell whether the signal quality will be better or worse than that of another site with similar interface impedance, or even if the site is on the fetus and not the mother.
Audio pulse tone generation has been employed in prior art pulse oximeters, an early example of which was described in U.S. Pat. No. 4,653,498. The prior art devices generate a simple tone for each identified pulse. In a further enhancement revealed in the patent, the tone pitch is proportional to the oxygen saturation level. Although useful for providing pulse rate and oxygen saturation trend information, this technique is inadequate for signal quality representation.
Sonification is the field of study dealing with the expression of information as humanly perceptible sound patterns. The human auditory system is highly sophisticated, featuring impressive dynamic range and parallel processing of many narrow sub-bands of the audible frequency range. This makes the audio medium ideal for expressing information that may contain subtle, time-varying features. Sonification has been suggested as a means of conveying more physiological information to the operator of a medical device, as described in U.S. Pat. No. 5,730,140. That patent teaches that the prior art in pulse tone generation (as cited above) suffers from the limitation of xe2x80x9cquantizationxe2x80x9d. That is, the complex, continuous signals acquired from the sensor have been reduced to artificial, simplistic beeps, with a drastic loss of information. The present invention seeks to avoid the information loss inherent in quantization.
The type of quantized pulse tone generation revealed in the prior art is dependent upon successful completion of a chain of algorithmic operations directed at accurately identifying pulsatile events in the input signals. With variations in design and implementation, analogous steps are performed in all pulse oximeters. The purpose of this signal filtering, pulse detection, and ratiometric computation is to obtain the pulse rate and oxygen saturation values, updating them on a relatively frequent basis (ideally, every pulse). These algorithms are generally tuned to rigorously avoid false positive pulse identification, as might occur during conditions of poor signal quality, since false pulses could result in erroneous pulse rate and/or oxygen saturation readings. Therefore, any condition compromising signal quality is likely to result in silence or merely a sporadic audio signal, without helping to discriminate the reason for the signal quality problem (movement, poor perfusion, low illumination level, etc.).
There remains a need for a pulse oximeter providing continuous signal quality information to the operator with rapid response to changing conditions at the sensor-tissue interface and little time delay. Such signal quality information would preferably be conveyed to the operator without requiring full attention to a visual display. That is, the operator should be capable of perceiving the signal quality information even while concurrently attending to the patient or manipulating the sensor. Preferably, the signal quality information would guide the operator during sensor adjustments intended to improve signal quality, that is, convey a spectrum of quality information rather than a quantized quality metric. The signal quality information would provide early warning of deteriorating signal quality before complete loss of pulse rate or oxygen saturation tracking. Lastly, the signal quality information should be made available in such a way that other information, for example, the physiological status of the patient and physiological or system alerts, can also be conveyed in parallel. The present invention meets these requirements by creating an audio signal based on sonification of the sensed signals.
It is therefore an object of the invention to provide a pulse oximeter with enhanced signal quality information to the operator.
It is another object of the invention to provide a pulse oximeter system in which signal quality may be represented continuously via sonification of the signals.
It is still another object of the invention to provide a pulse oximeter system in which further physiological information, such as oxygen saturation level, can be modulated into the same audio signal.
It is yet another object of the invention to provide a pulse oximeter system in which physiological or system alerts can be modulated into the same audio signal.
In the present invention, pulse oximetry is made more effective by continuously transforming the input signals from the sensor into an audio signal, augmenting or replacing visual representations of signal quality. This audio signal is available for the clinician""s use in guiding sensor placement even in the absence of successful computation of pulse rate and/or oxygen saturation level. Furthermore, various features of the audio signal are used to distinguish how good sensor placement may be, i.e., how robust the pulse detection will be if the sensing is perturbed by interfering factors such as pressure, slippage, etc.
The input signals from the sensor are typically indicative of the amount of light not absorbed by the blood-carrying tissue it has been transmitted though by at least two light emitting devices. Preferably, a number of steps are taken to condition the input signal and to accentuate the pulsatile components of the input signal before transforming the input signal to an audio signal.
Preferably, the input signals are initially conditioned to remove noise, due to ambient light, fluctuations on the input power line, drift, and high frequency interference. Other filters, for removing noise due to motion artifacts, or other sources, are also possible.
Once clean input signals are obtained, the filtered input signals can be further processed to accentuate the pulsatile components. For example, in one embodiment of the present invention, each of the input signals is differentiated with respect to time. The two signals are then merged and passed through a limiter which limits the amplitude of each pulse between pre-established xe2x80x9cfloorxe2x80x9d and xe2x80x9cceilingxe2x80x9d parameters indicative of a good quality signal. A xe2x80x9cclippedxe2x80x9d signal which is clipped off at both the floor and ceiling is indicative of high quality input data, while a signal that is not clipped is less reliable. Prior to transforming the signal to audio, the input signal can also be evaluated to determine xe2x80x9cpeakxe2x80x9d and xe2x80x9cvalleyxe2x80x9d levels at each pulse in the signal. These values can be used to dynamically modify the volume of the output audio such that a low volume signal indicates a lower quality signal as compared to a higher volume output. Furthermore, higher volume peaks and lower (or zero) volume valleys can be used to indicate systole and diastole phases of the heartbeat, respectively.
The resultant audio signal can be combined or merged with other audio alerts in the oximetry system. For example, in the event of a predetermined physiological alert condition or a system problem, the audio component of the oximeter can be constructed to provide an alert tone. Depending on the severity of the alert, the audio signal indicative of signal quality can be suppressed. Alternatively, the alert signal can be added to the audio signal.
Preferably, the signal transformation phase accentuates the pulsatile nature of the signals by a combination of filtering and differentiation in the time domain. As previously noted, the pulsatility of the photoplethysmographic waveform is quite distinctive and informative to the trained operator. Furthermore, interfering factors generally disturb this natural pulsatility, making it a natural feature to employ in assessing signal quality.
In another preferred embodiment of the invention, modulating the audio volume differently during systolic and diastolic phases of the signal further emphasizes the pulsatile nature of the input signals. In yet another preferred embodiment, one or more physiological parameters are encoded in the audio signal. In a highly preferred embodiment, the oxygen saturation is represented in the audio signal by adjusting the continuous tone frequency such that the peak frequency of the systolic phase of a particular pulsation represented in the audio signal is proportional to the recent oxygen saturation trend level.
In one highly preferred embodiment of the invention, the pulse oximeter system comprises a fetal sensor for monitoring oxygen saturation in the blood of a fetus while in the womb.
Other advantages and features of the invention, together with the organization and manner of operation thereof, will become apparent from the following detailed description when taken in conjunction with the accompanying drawings wherein like elements have like numerals throughout the drawings.